Bar code symbol scanner utilizing monitor photodiode of laser diode package as a photoreceiver

ABSTRACT

An optical reader/scanning system eliminates the need for and the use of a separate system receiver mounted exteriorly of a laser diode package. Instead, a monitor photodiode within the laser diode package is used as the system receiver. A fresnel lens increases the collection efficiency of the monitor photodiode. The fresnel lens may be mounted interiorly or exteriorly of the laser diode package.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 07/510,979, filed Apr. 18, 1990, and is related to U.S. patentapplication Ser. No. 07/193,265, now U.S. Pat. No. 5,144,120.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to an optical reader and an opticalscanning system of the type which directs a laser beam from a laserdiode package to a symbol for reflection therefrom, and which receivesreflected laser light at a discrete photoreceiver located exteriorly ofthe package and, more particularly, to a compact arrangement in suchreaders and scanning systems which eliminates the use of and the needfor such exterior discrete photoreceivers.

2. Description of Related Art

Various optical readers and optical scanning systems have been developedheretofore to optically read bar code symbols applied to objects inorder to identify the object by electro-optically reading the symbolthereon. The bar code symbol itself is a coded pattern comprised of aseries of bars of various widths and spaced apart from one another tobound spaces of various widths, said bars and spaces having differentlight-reflecting characteristics. The readers and scanning systemselectro-optically decoded the coded pattern to a multiplealpha-numerical digit representation descriptive of the object. Scanningsystems of this general type have been disclosed, for example, in U.S.Pat. Nos. 4,251,798; 4,387,297; 4,409,470; 4,845,350; 4,835,374 and4,816,660, all of which have been assigned to the same assignee as theinstant invention and are incorporated herein by reference thereto.

As disclosed in some of the above patents, a laser light beam wasgenerated by a semiconductor laser diode package. The laser beam wasdirected at, and repetitively scanned across, a symbol to be read, andthen at least a portion of the laser light reflected off the symbol wascollected and detected by a photodetector receiver. The photodetectorreceiver was a discrete component mounted separately from the laserdiode package. The photodetector receiver generated electrical analogsignals of variable amplitude which corresponded to the variableintensity of the light reflected off the symbol. The analog signals weredigitized and decoded into data descriptive of the symbol and, ofcourse, of the object bearing the symbol.

Although the known readers and scanning systems which utilized aseparate photodetector receiver were quite satisfactory in terms ofperformance, such use was disadvantageous in the context of making thereader/scanning system as light in weight, as small in size, as compact,and as inexpensive as possible. It would be desirable, particularly whenthe reader/scanning system was intended for handheld operation, toeliminate the use of a separate photodetector receiver withoutsacrificing reader/scanning system performance.

SUMMARY OF THE INVENTION 1. Objects of the Invention

It is a general object of this invention to advance the state of the artof readers and scanning systems operative for reading symbols havingparts of different light reflectivity such as bar code symbols.

It is another object of this invention to eliminate the use of, and theneed for, a separate photodetector receiver.

A further object of this invention is to provide a reader/scanningsystem of reduced weight, miniature size and low cost.

2. Features of the Invention

In keeping with these objects, and others which will become apparenthereinafter, one feature of this invention resides, briefly states, in asystem for reading symbols having parts of different light reflectivity.The invention comprises an arrangement including a conventional laserdiode package having laser chip means operative for emitting aforwardly-directed laser beam and a rearwardly-directed laser beam. Itis the forwardly-directed laser beam which is directed along an opticalpath toward a symbol for reflection therefrom to generate reflectedlight.

The conventional laser diode package also has monitor photodiode meansin optical communication with the rearwardly-directed laser beam. Themonitor photodiode means is operative for generating an output powercontrol signal for the laser chip means. The monitor photodiode means isactually part of a feedback loop and "watches" the power output of thelaser chip means. The monitor photodiode means, together with thefeedback loop, serve to maintain the power output of the laser chipmeans approximately constant.

In accordance with this invention, the monitor photodiode means is notonly used to monitor the power output of the laser chip means, as isconventional, but, in a departure from the prior art, is also used asthe photodetector receiver of the system. By using the monitorphotodiode means as the system receiver, the need for, and the use of, aseparate receiver are eliminated, thereby reducing the weight, size, andexpense of the overall system.

In a conventional laser diode package, the laser chip means has frontand rear facets optically aligned along an optical axis in the opticalpath. The forwardly-directed laser beam is concentric with this opticalaxis. The monitor photodiode means has a receiving opening which isconventionally spaced rearwardly behind the laser chip means and whichis radially offset from the optical axis.

Hence, in order to utilize the monitor photodiode means as the systemreceiver, it is necessary to provide the arrangement with collectormeans operative for collecting and optically modifying at least aportion of the reflected light from the symbol, and also for directingthe collected portion to the monitor photodiode means to generate aninformation signal descriptive of the symbol. The collector means mayadvantageously be one or more lenses or optical components in the returnpath of the reflected light. These optical components focus thecollected portion onto the offset and rearwardly-spaced receivingopening of the monitor photodiode means.

The use of the monitor photodiode means as the system receiver alsorequires that the arrangement be provided with signal processor meansfor separating the control and information signals, and for separatelyprocessing said signals. The separation of the control and informationsignals is necessary due to the "double duty" now required of themonitor photodiode means.

It will be noted that the control and information signals have differentelectrical characteristics. The output power control signal generated bythe monitor photodiode means, which is "watching" therearwardly-directed laser beam of the laser chip means, is essentially aDC signal, i.e. a voltage signal having a generally constant amplitude.On the other hand, the information signal generated by the monitorphotodiode means is an analog AC signal, i.e. a voltage signal having avariable amplitude. The DC and AC signals can be advantageouslyseparated by filter means, and separately processed. The AC informationsignal is thereupon digitized and decoded into data descriptive of thesymbol. The DC control signal is separately conveyed to a currentcontroller the fork loop operative, as described above, for controllinga power supply for the monitor photodiode means as a function of thesensed electrical current.

In one advantageous embodiment, wherein the system is used as a bar codesymbol scanner, focusing means are provided in the optical path fordirecting and optically modifying the forwardly-directed laser beam toform a beam spot on the symbol. Scanning means are also provided in theoptical path, and are operative for scanning the beam spot in a scanacross the symbol.

Advantageously, the focusing means includes a positive focusing lens andan aperture stop. The aforementioned collector means advantageouslyincludes a negative collecting lens having a clearance opening locatedin the optical path. The focusing means directs the forwardly-directedlaser beam through the positive lens, the aperture stop and theclearance opening prior to impinging on the symbol.

As for the return path along which the collected portion of thereflected light is directed, the negative collecting lens, incombination with the positive lens, are employed for directing thecollected light portion to the offset receiving opening of the monitorphotodiode means.

Rather than requiring the monitor photodiode means to perform "doubleduty" as described above, it is sufficient if the monitor photodiodemeans is used only for the "single duty" of detecting the collectedportion of the reflected light to generate the information signal. Theother duty of monitoring the power output of the laser chip means can beperformed by using the electrical characteristics of junction lasers. Anelectrical circuit monitors the AC voltage and the AC current of thelaser diode, and generates a bias current needed to operate the laserdiode above a threshold level independently of the temperature and ageof the laser diode. For details, reference can be had to "an AutomaticBias Control (ABC) Circuit for Injection Lasers", by A.Albanese, TheBell System Technical Journal, Vol. 67, No. 5, May-June 1978.

The aforementioned laser diode package, focusing means and the collectormeans are advantageously supported on and by a common support. Thesupport may be a hand-held wand, a stand-alone workstation, a built-inlaser scanning installation, or a hand-held laser scanning head. Thesupport may also be mounted on the aforementioned scanning means inorder to constitute a scan module of compact size that can be fittedinto a housing.

The novel features which are considered as characteristic of theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged, partly broken-away, front perspective view of alaser diode package;

FIG. 2 is an electrical schematic block diagram showing the electricalconnections for the laser diode package of FIG. 1;

FIG. 3 is a side sectional view of an arrangement according to thisinvention;

FIG. 4 is a diagrammatic top plan view of the arrangement of FIG. 3;

FIG. 5 is an exploded perspective view of the individual components ofFIG. 4;

FIG. 6 is a partly sectioned side view of a scanning system for readingbar code symbols in accordance with this invention:

FIG. 7 is a partly broken-away side sectional view of a reader forreading bar code symbols in accordance with this invention;

FIG. 8 is a side sectional view of another reader in which the presentinvention is incorporated;

FIG. 9 is a view analogous to FIG. 6 of a modified scanning system; and

FIG. 10 is another view analogous to FIG. 6 of another modified scanningsystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, reference numeral 10 generally identifiesa conventional laser diode package comprising a casing 12, a frontopening 14, and rear terminals 16, 18, 20 extending rearwardly of thecasing. A laser chip 22 having a front facet emits a forwardly-directedgenerally-conical laser beam 24 along an optical path through the frontopening 14 in a forward direction, as depicted by the arrow A. The laserchip also has a rear facet through which a rearwardly-directedgenerally-conical laser beam 26 is emitted into the casing 12 in acountercurrent direction to the arrow A. The front and rear facets arealigned along a common boresight or optical axis.

A monitor photodiode 28 is also mounted within the casing 12 rearwardlyof the laser chip 22, and also radially offset from the optical axis.The monitor photodiode 28 is, as shown in FIG. 2, part of a controlfeedback loop which monitors the output power of the laser chip andcontrols the output power to have a generally constant value.

Terminal 16 is connected to the cathode of the laser chip 22, and iselectrically grounded. The anode of the monitor photodiode 28 isconnected to terminal 18. A low pass filter 30 is electrically connectedto supply terminal 20 which is electrically connected to both the anodeof the laser chip and the cathode of the monitor photodiode.

As is conventional in laser diode packages, a DC current controller 32is electrically connected in the feedback loop between the terminal 18and the supply terminal 20. The monitor photodiode "watches" therearwardly-directed beam 26. When the current controller 32 senses adecrease in electrical current passing through the monitor photodiode28, the controller 32 increases the electrical current which isdelivered to the laser chip, and vice versa. In this way, the poweroutput of the laser chip is maintained essentially constant.

The laser diode package 10, as described so far, is entirelyconventional, and is used as a laser light source in systems of the typedisclosed in the aforementioned patents. Thus, as disclosed in, forexample, U.S. Pat. No. 4,816,660, the emitted forwardly-directed laserbeam 24 is directed through a positive focusing lens 34, e.g. aplano-convex lens, and an aperture stop 36. The focusing lens 34 andaperture stop 36 together serve to focus the laser beam 24 to a beamspot at a reference plane located a predetermined distance away from thelaser chip. As is conventional in this art, the symbol to be read islocated in the vicinity of this reference plane. In a scannerembodiment, a scanning means is operative for repetitively sweeping thebeam spot across the symbol in any desired scan pattern.

As also taught by the aforementioned patents, a separate receiver, e.g.one or more photodiodes, is separately mounted away from the laser diodepackage 10 to collect at least a portion of the light reflected off thesymbol. Now, in accordance with this invention, the use of one or moreseparate photodiodes is eliminated and, instead, the monitor photodiode28 within the laser diode package 10 is used as the light receiver fromwhich information relating to the symbol can be determined.

To use the monitor photodiode 28 as the receiver, the reflected light iscollected and directed to a receiving opening of the monitor photodiode.Since the monitor photodiode is deliberately spaced rearwardly of, andradially offset from, the laser chip to prevent light reflected from themonitor photodiode from getting back into the internal cavity of thelaser chip and causing noise in the laser output, a negative collectinglens 38 is positioned in the optical path in order to focus thereflected light onto the monitor photodiode. However, setting thenegative collecting lens 38 in the optical path will change the focallength of the positive focusing lens and, hence, interfere with theforwardly-directed beam 24 and change the focusing parameters of theoptical assembly.

To prevent changing the focusing parameters, a clearance passage 40centered on the optical axis is formed in and through the negativecollecting lens 38. The clearance passage 40 permits theforwardly-directed laser beam 24 to pass unobstructedly through and pastthe negative lens 38 prior to impingement on the symbol. In the returnpath, the returning reflected light 42 is optically modified by thenegative lens 38 in combination with the positive lens 34 to focus thereflected light 42 onto the monitor photodiode. The lenses 38, 34 havean equivalent focal length equal to the distance between the lens 34 andthe monitor photodiode.

As best shown in FIGS. 4 and 5, in a preferred embodiment, the size ofthe aperture in the aperture stop 36 is about 1 mm as considered along ahorizontal scan direction. According to previous proposals, the focusinglens 34 is circular and has a diameter of about 4 mm. The opticalproperties or focal length of the focusing lens in its central region,i.e. within a 1 mm area, around the optical axis are kept constant. Theexterior surrounding portions of the focusing lens can be provided withdifferent optical properties, e.g. a positive lens having a focal lengthequal to the distance between the focusing lens 34 and the monitorphotodiode.

As described so far, the rearward position of the monitor photodiode iscompensated for by either the negative collecting lens 38 together withthe focusing lens 34, or a focusing lens having a central region and anexterior surrounding region, each region having different opticalproperties. To compensate for the radial offset, the negative collectinglens 38 is asymmetrically positioned relative to the optical axis. Asbest shown in FIG. 3, the central axis 44 of the negative lens 38 isshifted relative to the optical axis.

The monitor photodiode 28 now performs two functions. The first is to"watch" the rearwardly-directed beam of the laser chip and generate apower output control signal. The second function is to detect aninformation signal descriptive of the symbol in a manner analogous to asystem photodetector of the prior art. Due to the superposition of thecontrol and information signals, this invention further compriseselectrical circuitry for separating the control and information signalsand for separately processing said signals.

The rearwardly-directed laser beam 26 is detected as a DC signal, i.e. avoltage having a constant amplitude. The reflected light 42 is detectedas an electrical analog AC signal, i.e. a voltage signal having avarying amplitude. The separation of the AC and DC signals isadvantageously achieved, as best shown in FIG. 2, by the low pass filter30 operative for allowing the DC signal to pass to the controller 32, aswell as a high pass filter 48 operative for allowing the AC signal topass to an analog-to-digital processor 50. The processor 50 digitizesthe AC signal into a digitized signal which, in turn, is conveyed to adecoder 52 which is operative for decoding the digitized signal intodata descriptive of the symbol. The analog-to-digital processor 50 anddecoder 52 are entirely conventional, and reference can be had to theaforementioned patents for further details.

The arrangement described herein, which eliminates the use of a separatesystem receiver outside of the laser diode package, is particularlybeneficial in the design of small and inexpensive readers and scanners.For example, as shown in FIG. 6, the laser diode package 10, positivelens 34, aperture stop 36, and negative lens 38 can advantageously bemounted in an optical tube 54 which, in turn, is mounted on a shaft 56of an oscillatable motor 58. A set screw 60 fixedly positions the tubeand the components supported thereby on the shaft 56. The oscillatablemotor 58 is, for example, identical to that disclosed in U.S. Pat. No.4,496,831. The oscillatable motor 58, of course, sweeps the tube 54 andthe components supported thereby in a reciprocatable cyclical manneracross a bar code symbol 60 to be read. The integrated assembly shown inFIG. 6 can advantageously be mounted in a housing 62. The housing 62 canbe built into a countertop as a permanent fixed installation, or can bemounted above a support surface to serve as a stand-alone workstation,or can be equipped with a handle for hand-held operation.

Another advantageous embodiment is shown in FIG. 7 wherein the laserdiode package 10, positive lens 34, aperture stop 36, and negative lens38 are mounted in an optical tube 64 which, in turn, is fitted onto anend of a wand 66. The wand 66 is dimensioned to conveniently fit withinone's hand, and is connected by means of a cable 68 to signal processingcircuitry. The wand 66 is conventionally manually dragged across thesymbol 60 to read the same.

The present invention may also be implemented in a hand-held,laser-scanning, bar code reader unit such as illustrated in FIG. 8. Thishand-held device of FIG. 8 is generally of the style disclosed in U.S.Pat. No. 4,760,248, assigned to the assignee of the instant application,and also is similar to the configuration of a bar code readercommercially available as part No. LS-8100 from Symbol Technologies,Inc. Alternatively, or in addition, features of aforementioned U.S. Pat.Nos. 4,387,297 and 4,409,470 may be employed in constructing the barcode reader unit of FIG. 8. U.S. Pat. No. 4,760,248 is incorporatedherein by reference.

An outgoing light beam 151 is generated in a reader unit 100, usually bya conventional laser diode package 146 or the like, and is directed toimpinge upon a bar code symbol a few inches from the front of the readerunit. The outgoing beam 151 is scanned in a fixed linear scan pattern,and a user positions the hand-held unit so that this scan patterntraverses the symbol to be read. Reflected light 152 from the symbol,although usually detected by a separate light-responsive device in thereader unit, is now detected by the monitor photodiode within thepackage 146. The monitor photodiode produces serial electrical signalsto be processed for identifying the bar code symbol.

The reader unit 100 is a gun-shaped device having a pistol-grip type ofhandle 153. A movable trigger 154 is employed to allow the user toactivate the light beam 151 and detector circuitry when pointed at thesymbol to be read, thereby saving battery life if the unit isself-powered. A lightweight plastic housing 155 contains the laser diodepackage, the optics and signal processing circuitry, and a centralprocessing unit (CPU) 140 as well as a battery 162. A light-transmissivewindow 156 in the front end of the housing 155 allows the outgoing lightbeam 151 to exit and the incoming reflected light 152 to enter. Thereader unit 100 is designed to be aimed at a bar code symbol by the userfrom a position where the reader unit 100 is spaced from the symbol,i.e. not touching the symbol or moving across the symbol. Typically,this type of hand-held bar code reader is specified to operate in therange of, perhaps, several inches.

As seen in FIG. 8, a suitable lens 157 (or multiple lens system) is usedto collimate and focus the outgoing laser beam onto the bar code symbolat an appropriate reference plane, and this same lens 157 may be used tofocus the incoming reflected light 152. The laser diode package 146 ispositioned to introduce the light beam along the axis of the lens 157toward an oscillating mirror 159 which is attached to a scanning motor160 that is activated when the trigger 154 is pulled. If the light beamproduced by the package 146 is not visible, an aiming light may beincluded in the reader unit. The aiming light may employ apartially-silvered mirror to introduce the light beam into the lightpath coaxially with the lens 157. The aiming light, if needed, producesa visible light spot which is scanned just like the laser beam. The useremploys this visible light beam to aim the reader unit at the symbolbefore pulling the trigger 154.

Although the present invention has been described with respect to linearor single line bar code symbols, it is not limited to such embodiments,but may also be applicable to stacked or two-dimensional bar codesymbols such as Code 49 and similar symbologies. It is conceivable thatthe present invention may also find application for use with variousmachine vision or optical character recognition applications in whichinformation is derived from other types of indicia such as characters orfrom the surface characteristics of the article being scanned.

In all of the various embodiments, the elements of the reader unit maybe assembled into a very compact package that allows the elements to befabricated as a single printed circuit board or integral module. Such amodule can interchangeably be used as the laser scanning element for avariety of different types of data acquisition systems. For example, themodule may be alternately used in a hand-held scanner, a table-topscanner attached to a flexible arm or mounting extending over thesurface of the table, or attached to the underside of the table top, ormounted as a subcomponent or subassembly of a more sophisticated dataacquisition system.

The module would advantageously comprise a laser/optics/photodetectorsubassembly mounted on a support and a scanning element such as arotating or reciprocating mirror. Control or data lines associated withsuch components may be connected to an electrical connector mounted onthe edge or external surface of the module to enable the module to beelectrically connected to a mating connector associated with otherelements of the data acquisition system.

An individual module may have specific scanning or decodingcharacteristics associated with it, e.g. operability at a certainworking distance, or operability with a specific symbology or printingdensity. The characteristics may also be defined through the manualsetting of control switches associated with the module. The user mayalso adapt the data acquisition system to scan different types ofarticles,or the system may be adapted for different applications byinterchanging modules on the data acquisition system through the use ofthe simple electrical connector.

The scanning module described above may also be implemented within aself-contained data acquisition system including one or more suchcomponents as a keyboard 148, a display 149, data storage 164,application software 166, and data bases. Such a system may also includea communications interface to permit the data acquisition system tocommunicate with other components of a local area network or with thetelephone exchange network, either through a modem or an ISDN interface,or by low power radio broadcast from the portable terminal to astationary receiver.

A fresnel lens 200 may be used to increase the efficiency of lightcollection. Thus, as shown in FIG.6, a fresnel lens 200 is mountedinside the casing 12 behind the front opening 14. Alternatively, asshown in FIG. 9, the fresnel lens surrounds the laser chip 22. As shownin FIG. 10, the fresnel lens is mounted outside the casing 12 in frontof the front opening of the casing 12. In each case, the fresnel lensfocuses the returning reflected light onto the monitor photodiode 28.The fresnel lens can be located anywhere in the path of the reflectedlight. The mounting of the fresnel lens inside the casing makes for avery compact scanning system. The mounting of the fresnel lens at thefront opening also helps to prevent contaminants from entering thecasing. The fresnel lens is particularly of benefit when used withvisible laser diodes.

It will be understood that each of the elements described above, or twoor more together, also may find a useful application in other types ofconstructions differing from the types described above.

While the invention has been illustrated and described as embodied in abar code symbol scanner utilizing monitor photodiode of laser diodepackage as a photoreceiver, it is not intended to be limited to thedetails shown, since various modifications and structural changes may bemade without departing in any way from the spirit of the presentinvention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this inventionand, therefore, such adaptations should and are intended to becomprehended within the meaning and range of equivalence of thefollowing claims.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

We claim:
 1. In a system for reading symbols having parts of differentlight reflectivity, an arrangement comprising:(a) a laser diode unithaving laser chip means operative for emitting a forwardly-directedlaser beam along an optical path toward a symbol for reflectiontherefrom to generate reflected light, and a rearwardly-directed laserbeam, said laser diode unit also having monitor photodiode means inoptical communication with the rearwardly-directed laser beam; (b)collector means including a fresnel lens for collecting and opticallymodifying at least a portion of the reflected light from the symbol, andfor directing the collected portion to the monitor photodiode means togenerate an information signal descriptive of the symbol; and (c) signalprocessor means for processing the information signal into datadescriptive of the symbol.
 2. The arrangement as recited in claim 1,wherein the monitor photodiode means is operative for generating anoutput power control signal for the laser chip means, and wherein thesignal processor means is operative for separating and separatelyprocessing the control and information signals.
 3. The arrangement asrecited in claim 1, wherein the unit has a housing bounding an interiorin which the laser chip means and the monitor photodiode means aremounted in a spaced-apart relationship.
 4. The arrangement as recited inclaim 3, wherein the laser chip means has front and rear facetsoptically aligned along an optical axis in the optical path, and whereinthe monitor photodiode means has a receiving opening radially offsetfrom the optical axis and spaced rearwardly behind the laser chip means.5. The arrangement as recited in claim 4; and further comprisingfocusing means for directing and optically modifying theforwardly-directed laser beam along the optical path to form a beam spoton the symbol, and means for scanning the beam spot in a scan across thesymbol.
 6. The arrangement as recited in claim 5, wherein the collectormeans directs the collected portion to the offset receiving opening ofthe monitor photodiode means.
 7. The arrangement as recited in claim 6,wherein the focusing means includes a positive focusing lens and anaperture stop, and wherein the collector means includes a negativecollecting lens having a clearance opening located in the optical path,and wherein the focusing means directs the forwardly-directed laser beamthrough the positive lens, the aperture stop and the clearance openingprior to impinging on the symbol.
 8. The arrangement as recited in claim7, wherein the negative lens together with the positive lens direct thecollected portion to the offset receiving opening of the monitorphotodiode means.
 9. The arrangement as recited in claim 2, wherein theoutput power control signal generated by the monitor photodiode means isa voltage signal having a generally constant amplitude; and wherein theinformation signal generated by the monitor photodiode means is avoltage signal having a variable amplitude; and wherein the signalprocessor means includes filter means operative for discriminatingbetween the generally constant voltage signal and the variable voltagesignal.
 10. The arrangement as recited in claim 9, wherein the signalprocessor means includes means for converting the variable voltagesignal to a digitized signal, and means for decoding the digitizedsignal into a decoded signal.
 11. The arrangement as recited in claim 9,wherein the signal processor means includes means for sensing electricalcurrent flowing through the monitor photodiode means, and means forcontrolling a power supply for the monitor photodiode means as afunction of the sensed electrical current.
 12. The arrangement asrecited in claim 1; and further comprising focusing means for directingand optically modifying the forwardly-directed laser beam along theoptical path to form a beam spot on the symbol; and wherein the laserdiode unit, the focusing means and the collector means are supported bya common support.
 13. The arrangement as recited in claim 12, whereinthe housing has a handle for hand-held operation.
 14. The arrangement asrecited in claim 13; and further comprising means for scanning the beamspot in a scan across the symbol; and wherein the laser diode unit, thefocusing means and the collector means are supported by the scanningmeans to constitute a compact laser scanning module.
 15. The arrangementas recited in claim 1, wherein the unit has a housing bounding aninterior, and wherein the fresnel lens is mounted within the interior ofthe housing.
 16. The arrangement as recited in claim 15, wherein thefresnel lens is mounted on, and surrounds, the laser chip means.
 17. Thearrangement as recited in claim 15, wherein the housing has a frontopening, and wherein the fresnel lens spans the front opening.
 18. Thearrangement as recited in claim 1, wherein the unit has a housing havinga front opening, and wherein the fresnel lens spans the front openingexteriorly of the housing.
 19. The arrangement as recited in claim 1,wherein the laser chip means is a visible laser diode.